In the area of particle formation, coating and granulation it is well known to use different techniques involving a fluidised bed apparatus. Basically, fluidisation is the operation by which solids are transformed into a fluid like state through suspension in a gas or liquid. If a fluid is passed upward through a bed of fine particles at a certain velocity, the frictional force between particles and fluid will just counterbalance the weight of the particles. In this state, the vertical component of the compressive force between adjacent particles disappears, and the pressure drop through any section of the bed about equals the weight of fluid and particles in that section. At this fluid velocity, the bed is just fluidised. Higher fluid velocities might lead to states where the fluid for example forms bubbles or gives rise to a turbulent motion in the solids. At even higher velocities, the fluid will entrain solids out of the bed and bring them along in its upward motion. The term fluidised bed is used when referring to different kinds of such beds.
When the fluid in a fluidised bed entrains large amounts of solid particles, a steady state can be achieved by collecting the entrained particles and returning them to the bed. Up to a certain amount of entrainment, a cyclone that is provided inside the vessel carrying the fluidised bed can provide the necessary collection and re-entry of particles for retaining steady state conditions in the vessel. Such a system is often referred to as a fluid bed. When bigger cyclone collectors must be provided outside the bed, such a system is often called a fast fluidised bed. Both these systems are circulating fluidised beds, and they are thus useful in different processes for particle treatment, as the particles will circulate several times in the system.
Within the pharmaceutical industry, fluidised beds are often used for granulation or coating of a product. Granulation is typically performed by spraying droplets of a liquid on particles, which are kept in the fluidised state. The particles will then tend to stick together, agglomerate, either by means of a binding agent in said liquid, or by a slightly dissolving effect from said liquid. Coating is usually performed by spraying a solution of coating agents onto the particles. In both granulation and coating processes it is important that conditions, such as the temperature, are such that the agglomerates or coatings will dry quickly enough so that unwanted agglomeration will not appear. On the other hand if drying of the agglomerates or coatings happens too quickly, unwanted spray drying of the material might appear.
An application of a circulating fluidised bed is described in the document U.S. Pat. No. 4,051,603, Kern, Jr. The fluidised bed apparatus revealed comprises a container wherein a fluidised bed is arranged. The particles are disposed above a perforated plate through which the fluid, in this case hot air, is allowed to pass. The entrained particles in the resulting fluidised bed are subjected to coating by means of a nozzle extending into the container and from which coating material is sprayed. The entrained particles and the fluid are sucked into a cyclone separator. The cyclone separator is provided in connection to the container and arranged to re-enter the entrained particles to the fluidised bed, and to lead off the remaining fluid. The particles will thus circulate from the bed to the separator and back. Each time a particle passes the nozzle, it will be sprayed with coating material and thus grow in size and weight. At a certain weight, the particle will be too heavy to be entrained with the fluid. Instead, it will fall down through the fluidised bed and be collected there. The process could of course be interrupted prior to that any particles reach such a size, and the particles collected directly from the bed. This kind of apparatus may be used for granulation instead of coating of particles. A granulation liquid will then be sprayed from a nozzle instead of a coating material.
The function of a cyclone separator is based on the action of centrifugal forces. These forces are produced by accelerating the fluid with the entrained particles onto a circular track provided around a vertical cyclone axis. The centrifugal force acting on each particle will increase with increasing particle size, so that larger particles will be flung against the wall of the separator and slide downwards to an outlet, which in this case would open towards the fluidised bed. Smaller particles will follow the fluid flow all through the separator and usually leave the cyclone through an opening in the separator ceiling. The specific grain size at which the particles start to separate from the fluid is often called the separating grain. The dimensions of the cyclone as well as what kind of fluid or particles used will be some of the parameters affecting the size of the separating grain.
Cyclone separators are widely used, although showing several disadvantages. Unevenly distributed coating on the particles may appear because of the swirls that will occur in the gas flow when passing the cyclone. Furthermore, the efficiency of a cyclone separator is low when separating very small particles, such as dust of spray-dried material from the coated/granulated particles. The dimensions of a specific cyclone offer only one separating grain, resulting in a narrow fraction of separated particles. Recycling must always be provided.
To avoid the problems related to the cyclone separators commonly used, several other devices have been proposed. In the document EP-B1-0 572 356 a fluidised bed apparatus is described provided with at least one vertical cartridge filter projecting into the fluidised bed container. The filter has a fabric filtration member, i.e. comprising a textile fabric. When such a filter has collected a certain amount of small particles, it must be cleaned unless the fluid flow through the filter will be severely restricted. In this case the filter is washed with a liquid cleaning agent. The filter is pushed upwards and out of the container in order to be washed. This is time-consuming and interrupts the operation of the fluid bed apparatus. In other documents, for example U.S. Pat. No. 5,766,281, Luy et al. and U.S. Pat. No. RE32 307, Glatt et al, different cleaning procedures are proposed, such as gas cleaning combined with wet cleaning or vibration cleaning of the filters. Filtration members made by paper or textile fabric have an additional disadvantage in that they have low strength and therefore they might be damaged when cleaned or blown out.
Particles clogging in the filter structure are a problem, which gives rise to the need of cleaning the filters. This is especially pronounced when dealing with small solid particles, such as unwanted dust of excess spray-dried material. Even if cleaning sometimes can be made during operation of the process, the fluid flows will be disturbed, and the overall efficiency will be decreased.
Other known techniques for particle separation are deflectors or fixed nets. For a fixed net, the separating grain is always decided by the mesh size, and problems with particles clogging the net do usually appear. The use of deflectors in for example a fluid passage can not be used for separating very small particles, since these will continue the travel in the passage along with the fluid. An additional end filter is therefore required for filtering off small particles.